The death toll soared to 748 people after authorities began counting deaths in the province surrounding Karachi, officials said Wednesday.
Authorities earlier reported 323 deaths in only Karachi in the three-day heat disaster.
But more deaths were reported by officials in Sindh Province, National Disaster Management Authority spokesman Ahmed Kamal said.
Officials also told CNN that the number of patients treated for heat stroke in Jinnah hospital, the largest in the Karachi, is 2,360.
The Sindh provincial government has declared a state of emergency in all its government hospitals, canceling leave of medical personnel and bringing in further medical supplies.
A report from CNN affiliate Geo.tv states that Sindh province-based troops from the Pakistan Rangers have set up ten heatstroke relief centers across the city.
The record-breaking temperatures would likely bring more deaths before cooler weather, forecast for later in the week, arrives.
Saturday`s temperature reached 44.8 degrees Celsius (112.64 degrees Fahrenheit) -- the highest-recorded temperature in Pakistan in the past 15 years.
Sunday`s temperature dipped slightly to 42.5 Celsius (108.5 F).
Ramadan obligations add pressure
Citizens in this predominantly Muslim country are observing Ramadan, the holy month when Muslim faithful around the world fast from sunup until sundown.
This means, that amidst these scorching temperatures, Pakistanis are foregoing food and water.
However, a noted cleric, Mufti Naeem, has said that certain people, including the elderly, the sick and children, should be excused from fasting if a doctor recommends it, Pakistani publication the Nation reports.
Making matters worse, Karachi is dealing with frequent power outages as the electricity grids are unable to keep up with the demand in the city of 20 million.
Acting President and Senate chair Mian Raza Rabbani has expressed his sorrow over the deaths.
A heat wave struck Pakistan`s neighbor India last month, killing more than 2,000 people.
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